(1) Field of the Invention
The present invention relates to an optical beam scanner which is applied, for example, to a laser beam printer.
(2) Description of the Related Art
A known optical beam scanner is schematically illustrated in FIG. 5. In this drawing, a scanner controller 1 is connected to a semiconductor laser diode 2 and a laser beam is emitted therefrom by application of a drive signal thereto. The drive signal is based on an ON-OFF signal from an external device (not shown) and a horizontal synchronized signal from a starting point (detection sensor described later). The laser beam emitted from the semiconductor laser diode 2 is collimated to a parallel wave beam by a collimator lens 3 and propagated to and impinged on one of the mirror surfaces of a rotary polygon mirror 5 driven by a scanner motor 4. The beam reflected by the mirror surface is passed through an f-.theta. lens 6 to equalize the scanning speed of the laser beam on a plane to be scanned. The laser beam is then irradiated onto a photosensitive drum 7 to be scanned, to form electrostatic latent images thereon.
In this scanning motion, the laser beam reflected by the polygon mirror 5 scans the drum 7 from left to the right in the drawing, in accordance with the rotation of the polygon mirror 5. The scanning laser beam is detected by a starting point detection sensor 8 through a mirror 9, which is disposed at a position corresponding to the left end of the photosensitive drum 7. The output detection signal from the sensor 8 is input to the scanner controller 1 to control the ON-OFF timing of the laser beam.
If the starting point detection sensor 8 is not accurately positioned at a predetermined vertical level, the sensor 8 can not detect the laser beam when the scanning beam is shifted vertically due to an unwanted inclination of the polygon mirror shaft away from vertical. This, reduces the reliability of the detection of the laser beam and causes a mis-synchronization of the scanning motion.
Therefore, conventionally, it has been necessary to provide a fine adjusting means for positioning the sensor 8 at a vertical level at which the luminous intensity of the scanning beam to be detected is at a maximum.
As described above, the conventional optical beam scanner requires a fine adjusting means for precisely positioning the sensor at a vertical level. Much time and labor are required for positioning the sensor, resulting in increased cost of the scanner and reduced reliability of the scanner.